Tantalum powder and method of making same

ABSTRACT

An improved flaked tantalum powder and process for making the flaked powder are disclosed. The powder is characterized by having a Scott density greater than about 18 g/in 3  and preferably at least about 90% of the flake particles having no dimension greater than about 55 micrometers. Agglomerates of the flaked tantalum powder, provide improved flowability, green strength and pressing characteristics compared to conventional flaked tantalum powders. The improved flaked tantalum powder can be made by preparing a flaked tantalum and then reducing the flake size until a Scott density greater than about 18 g/in 3  is achieved. The invention also provides pellets and capacitors prepared from the abovedescribed flaked tantalum powder.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 07/462,806 filed on Jan.10, 1990, abandoned, which is a division of application Ser. No.07/209,746 filed Jun. 21, 1988, now U.S. Pat. No. 4,940,490 and acontinuation-in-part of U.S. patent application Ser. No. 126,706, filedNov. 30, 1987, now abandoned.

FIELD OF INVENTION

The invention relates to flaked tantalum powders often used forelectrical capacitors and methods for making such powders. Moreparticularly, the invention relates to flaked tantalum powders whichwhen agglomerated provide the required electrical properties and goodprocessing properties, e.g., flowability, high green strength andpressability.

BACKGROUND OF THE INVENTION

Tantalum capacitors, made from tantalum powder, have been a majorcontributor to the miniaturization of electronic is circuits and havemade possible the application of such circuits in extreme environments.Tantalum capacitors typically are manufactured by compressingagglomerated tantalum powder to form a pellet, sintering the pellet in afurnace to form a porous tantalum body (electrode), and then subjectingthe porous body to anodization in a suitable electrolyte to form acontinuous dielectric oxide film on the sintered body.

Development of powders suitable for making tantalum capacitors hasresulted from efforts by both capacitor producers and tantalumprocessors to delineate the characteristics required for tantalum powderin order for it to best serve in the production of quality capacitors.Such characteristics include surface area, purity, shrinkage,pressability, green strength, and flowability.

First of all, the powder should provide an adequate surface area whenformed into a porous body and sintered. The μfV/g of tantalum capacitorsis proportional to the specific surface area of the sintered porous bodyproduced by sintering a tantalum powder pellet; the greater the specificsurface area after sintering, the greater the μfV/g. The specificsurface area of tantalum powder is related to the maximum μfV/gattainable in the sintered porous body.

Purity of the powder is an important consideration. Metallic andnon-metallic contamination tends to degrade the dielectric oxide film intantalum capacitors. While high sintering temperatures serve to removesome volatile contaminants high temperatures tend to shrink the porousbody reducing its net specific surface area and thus the capacitance ofthe resulting capacitor. Minimizing the loss of specific surface areaunder sintering conditions, i.e., shrinkage, is necessary in order toproduce high μfV/g tantalum capacitors.

Flowability of the tantalum powder and green strength (mechanicalstrength of pressed unsintered powder pellets) are also importantcharacteristics for the capacitor producer in order to provide efficientproduction. The flowability of the agglomerated tantalum powder isessential to proper operation of automatic pellet presses. Sufficientgreen strength permits handling and transport of a pressed product,e.g., pellet, without excessive breakage.

A `pellet`, as the term is used herein, is a porous mass or bodycomprised of tantalum particles. Green strength is a measure of apellet's mechanical strength. The term `pressability` describes theability of a tantalum powder to be pressed into a pellet. Tantalumpowder that forms pellets that retain their shape and have sufficientgreen strength to withstand ordinary processing/manufacturing conditionswithout significant breakage have good pressability.

Currently, tantalum powders suitable for use in high performancecapacitors are produced by several methods. One powder production methodinvolves chemical reduction, e.g., sodium reduction of potassiumfluorotantalate, K₂ TaF₇. In another method, powder is produced byhydriding a melted (typically arc melted or electron beam melted)tantalum ingot, milling the hydrided chips, and dehydriding.

As discussed above, the μfV/g of a tantalum pellet is a function of thespecific surface area of the sintered powder. Greater net surface areacan be achieved, of course, by increasing the quantity (grams) of powderper pellet; but, cost and size considerations have dictated thatdevelopment be focused on means to increase the specific surface area oftantalum powder.

One of the methods proposed for increasing the specific surface area oftantalum powder is flattening the powder particles into a flake shape.

Efforts to further increase specific surface area by making thinnertantalum flakes have been hindered by concomitant loss of processingcharacteristics. For example, several of the major deficiencies of verythin tantalum flake are poor flow characteristics, poor pressability andlow green strength and low forming voltages.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of this invention to provide a method for making flakedtantalum powder having a Scott density greater than about 18 g/in³.

It is another object of this invention to provide a flaked tantalumpowder wherein at least about 90% of the flakes have no dimensiongreater than about 55 micrometers and the individual flakes have asubstantially uniform cross section.

It is another object of this invention to provide a flaked tantalumpowder having a Scott density greater than about 18 g/in³ suitable foruse in economical high speed processes for the manufacture of tantalumcapacitors.

It is another object of this invention to provide an agglomerate offlaked tantalum powder that has good flowability and pressabilitycharacteristics.

It is another object of this invention to provide flaked tantalumpellets having high green strength.

It is another object of this invention to provide flaked tantalumpellets having reduced sensitivity to sintering temperatures, i.e.,pellets that can be sintered over a wide range of temperatures, relativeto the prior art, to form an electrode useful in a tantalum capacitor.

It is another object of this invention to provide a tantalum electrodehaving reduced sensitivity to forming voltages, i.e., dielectric oxidescan be formed on the electrodes over a range of voltages.

The present invention provides a method for making flaked tantalumpowder that, in agglomerated form, is flowable and pressable comprisingthe steps of preparing tantalum flakes and reducing the flake size sothat the resulting tantalum flake Scott density is greater than about 18g/in³. In one embodiment at least about 90% of the flakes have nodimension greater than about 55 micrometers, and in another embodimentno greater than about 25 micrometers. Preferably, at least about 90% ofthe flakes have no dimension greater than about 45 micrometers. Theflake may be embrittled, e.g., by hydriding, oxidizing, cooling to lowtemperatures or like methods to facilitate the flake size reductionstep.

The present invention provides a flaked tantalum powder produced from atantalum flake prepared from tantalum, powder produced by chemicalreduction methods. The flaked tantalum powders of this invention haveimproved processing properties, when agglomerated, including flowproperties suitable for high speed manufacturing operations and alsohave good pressability for forming high green strength pellets. Theflaked tantalum powder of the present invention has a Scott densitygreater than about 18 g/in³ preferably, in the range of about 18 g/in³to about 60 g/in³. A still more preferred range for the Scott density ofthe flaked tantalum powder is about 20 g/in³ to about 35 g/in³. A Scottdensity of about 21 g/in³ is most preferred. Preferably, at least about90% of the individual tantalum flakes have no dimension greater thanabout 45 micrometers and as such will pass through a 325 mesh screen.

The present invention also provides an agglomerate of theabove-described flaked tantalum powder having improved flowability andpressability characteristics. The agglomerated flaked tantalum powder ofthis invention may be prepared by any conventional method for preparingagglomerates such as, for example, by heating the tantalum flake,described in the preceding paragraphs, to a temperature of about 1300°to 1600° C. in an inert atmosphere or under vacuum for a period of about30 to 60 minutes and crushing the resulting product to a size of about40 mesh (0.015 inch screen opening).

The present invention also provides pellets prepared from the flakedtantalum powder described in the preceding paragraph.

The present invention also provides pellets prepared from the abovedescribed agglomerate of flaked tantalum powder.

The present invention also provides a capacitor electrode formed fromthe pellets described in the preceding paragraphs. In general thecapacitors are prepared by sintering the pellets described above andanodizing the sintered pellets.

Other details, objects and advantages of the invention and methods formaking and using the same will become apparent from the followingdetailed description and accompanying Figures. A legend at the bottom ofthe Scanning Election Micrograph (SEM) Figures gives the voltage,magnification e.g., 400X, and a reference scale in micrometers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a SEM at a magnification 1000X, of a prior art ingot-derivedflaked tantalum powder having a Scott Density of 13.4 g/in³ ;

FIG. 2 is a SEM at a magnification of 1000X, of an ingot-derived flakedtantalum powder having a Scott Density of 59.8 g/in³ prepared inaccordance with the present invention;

FIG. 3 is a SEM at a magnification of 400X, of a prior art flakedtantalum powder produced in accordance with Example II, specimen H ofU.S. Pat. No. 3,647,415 Yano, et al., and which is an example of atantalum flake that is not produced in accordance with the teachings ofthat patent;

FIG. 4 is a SEM at a magnification of 400X, of a flaked tantalum powderproduced by subjecting the flake of FIG. 3 to the process of the presentinvention;

FIG. 5 is a SEM at a magnification of 400X, of a prior art flakedtantalum powder produced in accordance with Example I, specimen C, ofthe U.S. Pat. No. 3,647,415 and which is representative of tantalumflake included within the teaching of that patent;

FIG. 6 is a SEM at a magnification of 500X, of a flaked tantalum powderproduced by subjecting the flake of FIG. 5 to the process of theinvention;

FIG. 7 is a SEM at a magnification of 1000X, of the flaked tantalumpowder shown in FIG. 3;

FIG. 8 is a SEM at a magnification of 1000X, of the flaked tantalumpowder shown in FIG. 4;

FIG. 9 is a SEM at a magnification of 1000X, of the flaked tantalumpowder shown in FIG. 5;

FIG. 10 is a SEM at a magnification of 1000X, of the flaked tantalumpowder shown in FIG. 6;

FIG. 11 is a graph showing the particle size distributions of the flakedtantalum powders shown in FIGS. 3 to 10;

FIG. 12 is a SEM at a magnification of 400X, of the flaked tantalumpowder shown in FIG. 3 after agglomeration;

FIG. 13 is a SEM at a magnification of 400X, of the flaked tantalumpowder shown in FIG. 4 after agglomeration;

FIG. 14 is a SEM at a magnification of 400X, of the flaked tantalumpowder shown in FIG. 5 after agglomeration;

FIG. 15 is a SEM at a magnification of 400X, of the flaked tantalumpowder shown in FIG. 6 after agglomeration;

FIG. 16 is a SEM at a magnification of 1000X, of the flaked tantalumpowder shown in FIG. 3 after agglomeration;

FIG. 17 is a SEM at a magnification of 1000X, of the flaked tantalumpowder shown in FIG. 4 after agglomeration;

FIG. 18 is a SEM at a magnification of 1000X, of the flaked tantalumpowder shown in FIG. 5 after agglomeration;

FIG. 19 is a SEM at a magnification of 1000X, of the flaked tantalumpowder shown in FIG. 6;

FIGS. 20A and 20B are photographs of pellets pressed from theagglomerated flaked tantalum powder shown in FIGS. 12, 13, 16 and 17;and

FIGS. 21A and 21B are photographs of pellets pressed from theagglomerated flaked tantalum powder shown in FIGS. 14, 15, 18 and 19.

DETAILED DESCRIPTION OF THE INVENTION

Flaked tantalum powder may be prepared by deforming or flattening agranular tantalum powder. It will be appreciated by those skilled in theart that this deformation may be accomplished by conventional mechanicaltechniques using a ball mill, rod mill, roll mill or the like. Theflaked tantalum powder of the present invention can be prepared fromsuch conventionally prepared flaked tantalum powder by reducing the sizeof the flake particles until a Scott density greater than about 18 g/in³is achieved. Preferably, this size reduction process may be aided byembrittling the conventional flake by techniques such as hydriding,oxidizing, cooling to low temperatures, or the like, to enhance breakagewhen reducing the flake particle size by mechanical means such ascrushing, or other size reduction processes.

In the present invention flakes are reduced in size withoutsubstantially reducing the thickness or tapering the peripheral edges ofthe flakes. Consequently, in one embodiment the flaked tantalum powderof the invention is characterized by flakes of substantially uniformthickness from edge to edge. Moreover, these flakes may be thicker thanprior art flakes of similar size, e.g., at least 90% of the flakeshaving no dimension greater than about 45 micrometers. The increasedthickness is demonstrated by BET nitrogen surface area values which aretypically less than about 0.7 m² /g and preferably in the range of about0.4 m² /g to 0.6 m² /g and more preferably about 0.5 m² /g. An importantadvantage attributable to the thicker flake is the ability to beanodized to higher forming voltages.

Comparison of FIGS. 1 thru 10 illustrates that the flaked tantalumpowder of the present invention is comprised of substantially smallerparticles than the prior art flakes. The tantalum flake shown in FIG. 1was prepared from classified 20×44 micrometer ingot-derived (electronbeam melted) tantalum chips. The chips were degassed in a vacuum furnaceto remove hydrogen and sieved through a 325 mesh screen. The resultingmaterial was then milled in a vibrating ball mill for 10 hours toflatten the chips into flake. This flake was acid leached first in aHCl/HNO₃ mixture and then in HF to remove metallic impurities. Theresulting flake which had a Scott density of 10.8 g/in³ was heat treatedat 1600° C. for 30 minutes to produce an agglomerated material which wasthen jaw crushed to 40 mesh sized agglomerates having a Scott density of13.4 g/in³.

The flaked tantalum powder shown in FIG. 2 was prepared fromingot-derived (electron beam melted) 325 mesh tantalum chips. The chipswere degassed in a vacuum furnace to remove hydrogen and sieved througha 325 mesh screen. The resulting material was milled in a vibrating ballmill for 10 hours to flatten the chips into flake. This flake was acidleached first in a HCl/HNO₃ mixture and then in HF to remove metallicimpurities. The resulting flake had a BET nitrogen surface area value of0.38 m₂ /g and a Scott density in the range of 10 to 15.2 g/in³. Thisflake was hydrided and subjected to cold isostatic pressing at 30,000psi to break the flake into smaller pieces which after pressing are inthe form of a solid bar. The solid bar was jaw crushed to 60 meshproducing flakes having a BET nitrogen surface area value of 0.54 m² /gand Scott density of 59.8 g/in³.

Comparison of the flaked tantalum powder shown in FIGS. 1 (prior art)and 2 (invention) demonstrates that the flake of this invention iscomprised of substantially smaller flake particles.

The prior art flaked tantalum powder shown in FIGS. 3 and 7 was madefrom -60 mesh sodium reduced tantalum powder. The powder was deformed toflake shape by milling in a vibratory ball mill for ten hours. The ballmilled flakes were acid leached to remove metallic impurities using 15%HCl and 2% HF. This method corresponds to the procedure described inU.S. Pat. No. 3,647,415 for preparing Specimen H in Example II. TheScott density of the resultant flake was 12.54 g/in³ and 90% of theflakes had no dimension larger than 126 micrometers as shown in Table 1.

The flaked tantalum powder of this invention shown in FIGS. 4 and 8 wasmade from -60 mesh sodium reduced tantalum powder. The powder wasdeformed to flake shape by milling in a vibratory ball mill for tenhours. The ball milled flake was acid leached to remove metallicimpurities using 15% HCl and 2% HF. The flake was then heated in aclosed vessel until the flake reached about 850° C. Then, however, theheated tantalum flake was hydrided by allowing it to cool to roomtemperature in the vessel while a positive hydrogen pressure of +5 psiwas maintained. The hydrided flake was reduced in size by milling theflake material in a Vortec Ml impact mill, available from VortecProducts Co., Long Beach, Calif., U.S.A., operating at 10,000 rpm. Theresultant flake had a Scott density of 21.45 g/in³ and about 90% of theflakes had no dimension greater than about 37 micrometers.

The prior art flaked tantalum powder shown in FIGS. 5 and 9 was alsomade from -60 mesh sodium reduced tantalum powder. This powder had anabsorbed hydrogen content of about 125 ppm. The powder was deformed toflake shape by milling in a vibratory ball mill for six hours. Then theball milled flake was acid leached to remove impurities using 15% HCland 2% HF. The resultant flake had a Scott density of 12.7 g/in³ andabout 904 of the flakes had no dimension greater than about 131.8micrometers. This method corresponds to the procedure described in U.S.Pat. No. 3,647,415 for preparing Specimen C in Example I.

The flaked tantalum powder of this invention shown in FIGS. 6 and 10were made from -60 mesh sodium reduced tantalum powder. The powder wasdeformed to flake shape by milling in a vibratory ball mill for sixhours. The ball milled flake was acid leached to remove metallicimpurities using 15% HCl and 2% HF. The flake was then heated in aclosed vessel until the flake reached about 850° C. Then, however, theheated flake was hydrided by allowing it to cool to room temperature inthe vessel while a positive hydrogen pressure of +5 psi was maintained.The hydrided flake was reduced in size by milling the flake material ina Vortec Ml impact mill operating at 12,500 rpm. The resultant flake hada Scott density of 28.30 g/in³ and about 90% of the flakes had nodimension greater than about 23.2 micrometers.

It is apparent from FIG. 11 and the Granulometer data in Table 1 thatthe particle size of the flake of this invention is substantiallysmaller than the particle size of the flake of the prior art. It is alsoclear that the particle size distribution of the flaked powder of theinvention is narrower than the particle size distribution of the flakeof the prior art.

The screen size distribution, distribution of mean particle size andScott density of the above-mentioned flaked powders were measured asdescribed below. The results obtained are set forth in Table 1.

Screen Size Distribution

To determine the screen size distribution of flaked tantalum powder, a325 mesh stainless steel screen having a screen opening of 45micrometers (ASTME-11 Specification), and a 500 mesh stainless steelscreen having a screen opening of 25.4 micrometers both screens having adiameter of 21 centimeters and manufactured by W. S. Tyler Corporation)are cleaned and dried to constant weight. The dried screens are tared tothe nearest 0.01 g. The tared screens are stacked with the 325 meshscreen placed above the 500 mesh screen.

A 20 g sample of flake is weighed to the nearest 0.01 g and placed ontothe 325 mesh screen. A stream of deionized water from a 0.25 inchinternal diameter tube is directed at the sample on the 325 mesh screenat a rate of 2 liters per minute until a total of 6 liters of deionizedwater is used to facilitate the screening process. The 325 mesh screenis then removed from the stack and deionized water is directed to thesample remaining on the 500 mesh screen in the same manner as describedabove. During both screening operations the flow of the water isdirected against the flake to most effectively cause the flake particlesto pass through the screens. Both screens including remaining sample arethen rinsed with methanol and dried at a temperature of 70° C. toconstant weight.

The dried screens are cooled to room temperature and weighed to thenearest 0.01 g. The weight of the sample retained on the 325 mesh screenand the sample retained on the 500 mesh screen are calculated bysubtracting the screen tare weight from the respective final grossweights. The various percentages reported in Table I are calculated fromthe data obtained by this procedure.

As shown by the data in Table I the flaked tantalum powder of theinvention is primarily comprised of particles having a size that willpass through the 500 mesh screen, i.e., particles having no dimensiongreater than about 25.4 micrometers. On the other hand, the size of theprior art flaked tantalum powder is much larger as demonstrated by thefact that most of the particles thereof were retained on the 325 meshscreen, i.e., particles having a dimension greater than about 45micrometers.

Distribution of Mean Particle Size

The mean particle size distribution of samples of flaked tantalum powderare determined using a model 715 Granulometer. This is an apparatusdesigned to make granulometric measurements on pulverulent productssuspended in a liquid. By means of a self-contained computer, theapparatus very quickly determines the distribution of mean particlesizes in a range from 0 to 192 micrometers. The sample numbers appearingin Table I (F3 through F6) correspond to the flakes shown in FIGS. 3through 6. The results of the Granulometer particle size distributionare set forth in Table I and are graphed in FIG. 11. In FIG. 11, thesolid lines reference the flaked tantalum powder of the subjectinvention shown in FIGS. 4 and 6 and the dotted lines reference theprior art flaked tantalum powder shown in FIGS. 3 and 5.

A review of the data in Table 1 reveals that the particle size of theflakes of the present invention are substantially smaller than those ofthe prior art flakes, which affirms the findings of the screendistribution measurements. Further, the curves of FIG. 11 show that theparticle size distribution of the flakes of the present invention aremuch narrower than those of the prior art flakes.

Scott Density

Scott density is determined with an apparatus comprised of a powderflowmeter funnel, a density cup and stand for the funnel and cupavailable as a set from Alcan Aluminum Corp., Elizabeth, N.J., U.S.A.The measurement is made by pouring a flake sample through the funnelinto the cup (one-cubic-inch nickel plated) until the sample completelyfills and overflows the periphery of the cup. Then the sample isleveled-off by means of a spatula, without jarring, so that the sampleis flush with the top of the cup. The leveled sample is transferred to abalance and weighed to the nearest 0.1 gram. The Scott density is theweight per cubic inch of the sample.

As Table I shows the flakes of the invention have Scott densities ofabout twice that of the prior art flakes tested.

                  TABLE I                                                         ______________________________________                                                       F3               F5                                                           (Prior           (Prior                                        Sample No.     Art)     F4      Art)   F6                                     ______________________________________                                        Screen Distribution                                                           (% Sample retained)                                                           % +325         61.40    3.34    67.88  1.59                                   % -325/500     13.08    4.19    8.45   1.24                                   % -500         25.52    92.47   23.67  97.17                                  Scott Density  12.54    21.45   12.70  28.30                                  (g/in.sup.3)                                                                  Granulometer Summary+                                                         (D) 10%*       33.5     6.7     41.6   4.6                                    (D) 50%**      83.5     17.4    93.2   11.1                                   (D) 90%***     126.5    37.0    130.8  23.2                                   ______________________________________                                         +As measured on Compagnie Industrielle Des Laser, Cilas Alcatel               Granulometer Model 715.                                                       *Length in micrometers that is greater than the measured diameter of the      smallest 10 percent by volume of the particles in the sample.                 **Length in micrometers that is greater than the measured diameter of the     smallest 50 percent in volume of the particles in the sample.                 ***Length in micrometers that is greater than the measured diameter of th     smallest 90 percent by volume of the particles in the sample.            

Agglomeration

The flaked tantalum powders are agglomerated in any conventional mannerin order to provide a product which is suitable for subsequent formationinto pellets from which capacitor electrodes can be fabricated.Typically, agglomeration involves heat treatment of the flake in avacuum or inert atmosphere at temperatures in the range of about 1300°to 1600° C. for periods of time ranging from about 30 to 60 minutes. Thespecific agglomeration technique utilized herein is described below.

The agglomerates shown in FIGS. 12 through 19 were made utilizing theflakes shown in FIGS. 3, 4, 5 and 6. The flakes of FIGS. 3, 4, 5 and 6were individually heat treated under vacuum at 1440° C. for 30 minutesand jaw crushed to -40 mesh. The flakes were doped with 100 ppmphosphorus and then subjected to a second heat treatment at 1500° C. for30 minutes and jaw crushed to -40 mesh. The resulting flakes weredeoxidized by blending with 2.5% Mg powder and heating under +3 psiArgon pressure to temperature of 950° C. for 320 minutes. The resultingagglomerates were acid leached, to remove MgO and excess Mg, using 15%HNO and then rinsed and dried.

The flow rates of the agglomerates shown in FIGS. 12 through 19 weremeasured according to ASTM Test Method B213-83. The agglomerates of theinvention as shown in FIGS. 13 and 17 flowed at a rate of 0.72 grams persecond and the agglomerates of the invention as shown in FIGS. 15 and 19flowed at 0.84 grams per second. On the other hand, the agglomerates ofthe prior art as shown in FIGS. 12, 14, 16 and 18 did not flow at alland thus the flow rate could not be measured. Failure to flow is verydetrimental since it is impractical to commercially press pellets fromagglomerates that do not flow.

Pellet Fabrication and Crush Strength

An agglomerated flaked tantalum powder is compressed in a conventionalpellet press without the aid of binders using an imbedded tantalum wire.Two samples of a tantalum powder, one weighing 1.29 g and the otherweighing 1.33 g are separately introduced into a die of a pellet presshaving a diameter of 0.250 inch. The press is set up to press a pellethaving a length of 0.330 inch. Utilizing the above weights and lengthsan approximate green density of 5.0 g/cc is achieved.

Pressability and green strength improvements exhibited by the flakedtantalum powder of this invention are apparent from the photographs ofFIGS. 20 and 21. FIGS. 20 and 21 show pairs of pellets pressed from aflake of the invention in comparison to pellets prepared from the priorart flake. FIG. 20 compares the pellets pressed from the flakes of FIGS.12 and 13. FIG. 21 compares the pellets pressed from the flakes of FIGS.14 and 15. In each pair, the pellet on the left is pressed from flake ofthis invention while the pellet on the right was prepared from the priorart flake.

The pellets prepared with the prior art flakes, expanded when releasedfrom the die such that their lengths were irregular and could not beaccurately measured. As can be seen from FIGS. 20 and 21 the pelletsprepared from prior art flake were also deformed and cracked. Incontrast, pellets made from flake of the invention maintained thespecified length and were suitable for further testing and processinginto anodes. Usable pellets could not be made from the prior art flaketested; whereas, the flake of this invention produced usable pelletswhich retained the desired shape and have crush strength adequate forutilization in manufacturing tantalum capacitors.

Although we have set forth certain present preferred embodiments of ourflaked tantalum powder and methods of making same, it should bedistinctly understood that our invention is not limited thereto but maybe variously embodied within the scope of the following claims.

We claim:
 1. A method for making a flaked tantalum powder, comprisingthe steps of:preparing tantalum flake powder, and reducing the flakesize by fracturing, without substantially reducing the thickness ortapering the peripheral edges of the flakes, until the powder has aScott Density greater than about 18 g/in³.
 2. The method of claim 1,wherein the flake size is reduced until the Scott density is in therange of about 18 g/in³ to about 60 g/in³.
 3. The method of claim 1,wherein the flake size is reduced until the Scott density is in therange of about 20 g/in³ to about 35 g/in³.
 4. The method of claim 3wherein the reduced size flakes have a BET surface area value of about0.5 m² /g.
 5. The method of claim 1, wherein the flake size is reduceduntil the Scott density is about 21 g/in³.
 6. The method of claim 1,wherein at least about 90% of the reduced size flakes have no dimensiongreater than about 25 micrometers.
 7. The method of claim 6, furthercomprising the step of embrittlement of the flaked tantalum powder priorto the size reduction step.
 8. The method of claim 7, wherein theembrittlement step is accomplished by hydriding.
 9. The method of claim6, further comprising the step of agglomerating the reduced size flake.10. The method of claim 6 wherein the reduced size flakes have a BETsurface area value of about 0.4 m² /g to 0.6 m² /g.
 11. The method ofclaim 1, wherein at least about 90% of the reduced size flakes have nodimension greater than about 55 micrometers.
 12. The method of claim 11,further comprising the step of embrittlement of the flaked tantalumpowder prior to the size reduction step.
 13. The method of claim 12,wherein the embrittlement step is accomplished by hydriding.
 14. Themethod of claim 11, further comprising the step of agglomerating thereduced size flake.
 15. The method of claim 11, wherein the tantalumflake is prepared from a tantalum powder produced by a chemicalreduction process.
 16. The method of claim 11, wherein the tantalumflake is prepared from a tantalum ingot.
 17. The method of claim 1,wherein at least about 90% of the reduced size flakes have no dimensiongreater than about 45 micrometers.
 18. The method of claim 17, furthercomprising the step of embrittlement of the flaked tantalum powder priorto the size reduction step.
 19. The method of claim 18, wherein theembrittlement step is accomplished by hydriding.
 20. The method of claim17, further comprising the step of agglomerating the reduced size flake.21. The method of claim 17 wherein the reduced size flakes have a BETsurface area value of about 0.5 m² /g.
 22. The method of claim 1,wherein the tantalum flake is prepared from a tantalum powder producedby a chemical reduction process.
 23. The method of claim 1, wherein thetantalum flake is prepared from a tantalum ingot.
 24. The method ofclaim 1 wherein the reduced size flakes have a BET surface area value ofabout 0.4 m² /g to 0.6 m² /g.